Exobase (source code)
= Exobase
{title2=$\ell\sim H$}
The exobase marks the transition between a collisional atmosphere and its <exosphere>. For a neutral, approximately hydrostatic <ideal gas>, take <mean free path> $\ell\simeq1/(n\sigma_c)$ and <atmospheric scale height> $H=k_BT/(mg)$. The criterion $\ell\sim H$ gives
$$
n_e\sim\frac1{\sigma_c H},\qquad P_e\sim\frac{mg}{\sigma_c}.
$$
An order-one collision convention can change this coefficient. The <pressure> is not universal: the <collision cross-section>, composition, local gravity, and ionization must be specified. A strongly escaping, non-hydrostatic atmosphere requires an appropriate density-gradient length instead of a hydrostatic $H$.
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